The Role of Iron-Hyponitrite Intermediates in Biology and Insights From Synthetic Model Complexes

Nitric oxide reductases (NORs) are a class of metalloenzymes that are known to mediate the 2 e^-/ 2 H⁺ reduction of nitric oxide (NO) to nitrous oxide (N₂O). There are three known types of NOR enzymes, each using a different iron-based active site to mediate NO reduction. Despite the difference in active site architecture across NORs, NO reduction is expected to be linked by a common intermediate: hyponitrite (N₂O₂ ^2-). However, experimentally, very little is known about the expected Fe-hyponitrite intermediates. In this Perspective, we provide a detailed discussion of Fe-hyponitrite chemistry. We start with an introduction into NOR metalloenzymes and provide a detailed account of the known and proposed steps of catalytic NO reduction by these enzymes with a specific focus on the proposed Fe-hyponitrite intermediates. We then turn our focus on what is known about synthetic Fe-hyponitrite complexes. We introduce hyponitrite as a ligand and discuss sources of preformed hyponitrite. We then introduce the three known Fe-hyponitrite complexes and discuss how these relate to NO reduction in NORs. Finally, we summarize what is known about Fe-hyponitrite chemistry and give an outlook on the aspects of this chemistry that are still vastly underexplored.